This invention relates generally to circuits known as charge pumps and, more particularly, to charge pumps as used in conjunction with phase-locked loops (PLLs). Phase-locked loops are widely used in a variety of applications in which a time-varying signal must be synchronized (locked) with a reference signal. Typical applications include frequency synthesizers for radio receivers and transmitters, demodulation of radio signals, and recovery of clock timing information from received radio signals or from digital storage devices. A common PLL configuration includes a phase detector, a charge pump, a loop filter, and a voltage controlled oscillator, the output of which is fed back as an input to the phase detector. The theory of operation of PLL circuits that include a charge pump is well known in the art. Basically, the phase detector generates signals indicative of the phase difference between a time-varying signal and a reference signal. Then the charge pump, controlled by output signals generated by the phase detector, adds (or subtracts) a controllable amount of charge, i.e., current, to (or from) an output signal that it generates. This output signal is smoothed by the loop filter and then used to provide a voltage that controls the voltage controlled oscillator.
A conventional charge pump in the context of a PLL consists of a number of current switches, which are turned on and off under control of signals generated by the phase detector. Conceptually, the charge pump is sometimes described as being part of the phase detector, but for purposes of the present invention the charge pump is best considered a separate device. Charge pump circuits for PLL applications are extremely critical if low noise operation is desired. Conventional charge pump circuits contain multiple current sources that are switched on or off depending upon the desired pump output current. These switching operations necessarily result in abrupt changes in total current drawn by the charge pump circuit, and these abrupt current changes result in noise components that can modulate onto the power supply itself and adversely affect other sensitive components of the PLL.
Accordingly, there is a need for an improved charge pump circuit, or an adjunct to an existing charge pump circuit, which eliminates these switching noise components and enhances the overall performance of the PLL in which the charge pump circuit is employed. The present invention is directed to this end.